Deportivo forked out £10m to bring Sergio across the country from Espanyol and his attacking displays look to have covered the cost.
Having been at Espanyol for six years, he struggled to find his feet at Deportivo immediately.
Playing for a higher profile side looks to have given his international career the required boost and he picked up his first cap in qualifying.
Coach Jose Antonio Camacho is a keen follower of his resilient, hard-graft attitude in midfield but he could face a battle for a starting place.